Data storage devices including, e.g., those normally provided as part of, or in connection with, a computer or other electronic device, can be of various types. In one general category, data is stored on a fixed or rotating (or otherwise movable) data storage medium and a read head, a write head and/or a read/write head is positioned adjacent desired locations of the medium for writing data thereto or reading data therefrom. One common example of a data storage device of this type is a disk drive (often called a “hard” disk or “fixed” disk drive). Although many concepts and aspects pertaining to the present invention will be described, herein, in the context of a disk drive, those with skill in the art, after understanding the present disclosure, will appreciate that the advantages provided by the present invention are not necessarily limited to disk drives.
In one type of disk drive, the medium is one or more rotatable disks having magnetizable coatings and rotatable about a rotation axis. An arm is mounted to position the tip of the arm anywhere along an arcuate path which intersects substantially all radial positions of the data recording surface of the disk. The arm tip, which bears a read head, a write head and/or a read/write head is mounted and configured in such a way that (owing at least in part to aerodynamic effects) the tip is substantially maintained spaced from, but relatively close to (such as on the order of a few nanometers) the data-bearing surface of the disk. In most configurations it is desirable to avoid or eliminate instances of contact with data-bearing surfaces of the disk (although contact may be provided at other places and times such as during disk spin-up or spin-down, parking operations, non-data-bearing areas, and the like). Data and/or servo information is provided to disk control (or other) circuitry or components and/or to a computer or other host device, e.g., using flex circuits and the like.
Although manufacture, distribution and use of disk drives follow a number of models, in at least some cases, following assembly of a disk drive, one or more testing procedures are performed. Often, testing is provided which is intended to identify, before they are distributed to users, any disk drives which may exhibit performance or reliability issues, such as having an unacceptably high bit error rate (BER) or other error measure. Because drive failure rates during such testing are correlated with factory costs, it would be advantageous to provide a method and/or apparatus which can reduce the failure rates.
In addition to reliability/performance testing, environmental testing may be performed. In some situations, environmental testing includes measuring and/or storing data related to how certain aspects of the disk drive react to various temperatures, pressures or other environmental factors. For example, environmental testing may be used to store information to control the magnitude of write head current as a function of ambient temperature (e.g., since a higher write current may be needed before the disk drive has “warmed up”). Typically, such ambient temperature corrections are performed on a relatively long time scale such as longer than a typical continuous write operation.
Other operations may be performed prior to normal use of the disk drive (i.e., prior to use for reading and/or writing data sent to, or received from, the computer or other host device). In one such operation, the disk is provided with sector markers or identifiers and track markers or identifiers (referred to herein as “location information”). Location information is generally distinguishable from data at least in that location information is typically used for purposes of positioning the read/write heads (typically, internally to the disk drive) while data is received from or sent to the computer or other host device. The location information (or “servo” information) is typically provided on the disk prior to any normal use of the disk. Further, as a general rule, the location information, once it is provided on the disk, is not, thereafter, altered or erased while, in most disk drives, data can be erased or written-over (although in some applications, some or all portions of disks may be designated as “read-only”).
The general trend in data storage, including disk drives, has been for increasingly higher data density on the medium. Higher densities permit not only construction of a physically smaller data storage device, for a given capacity, but also can assist in enhancing performance (reducing seek times, and the like). As will be understood by those skilled in the art, increases in data density are often associated with reductions in the distance between the read/write head and the medium (reduction in “flying height”). Among the technical difficulties encountered when attempting to reduce flying height, a dilemma arises which is related to the pole tip protrusion (PTP) phenomenon. During a write operation, current is provided to the write head. Write current causes an amount of heating of the read/write head or arm tip, which, in turn, causes thermal expansion resulting in protrusion of the tip towards the surface of the medium. It has been found that, at moderately high write currents, the amount of thermal expansion of the arm tip is so large that, for prolonged write operations (e.g., substantially continuous writes over multiple disk segments), the tip may protrude sufficiently to contact a medium. This is an especially large risk if the nominal flying height of the head is small. On the other hand, if the magnitude of the write current is reduced, in an attempt to avoid contact, the tip (and thus the write head) during the first portion of the write operation (i.e., before the tip has substantially heated and expanded) may be too far from the medium, such as being so far from the medium that, at the reduced write current, the magnetic flux provided by the write head is insufficient to saturate the medium, and thus is insufficient to write or erase data, as desired.
Accordingly, it would be desirable to provide a method and apparatus which can achieve a low flying height while both providing magnetic saturation throughout the write operation and avoiding contact with the data medium throughout the write operation.